Bubble Nucleation in Stout Beers (Image 3)

This image, part of a series, was taken from a video showing cellulose fibers submerged in stout beer, observed under a microscope. Pockets of air trapped inside the fibers become seed bubbles that trigger nucleation (the formation of additional bubbles). Here, the bubble continues growing ever larger due to flux of nitrogen and carbon dioxide gas through the walls of the cellulose from the surrounding liquid. [Image 3 in a series of 6 related images. See Image 4.]

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Research by Michael Devereux, a graduate student in the department of mathematics and statistics at the University of Limerick in Ireland, found that microscopic plant fibers can be used to froth Stout as well as a widget, the device developed by Guinness that's currently used to form the frothy, creamy head in many stout brews.

Widgets are small plastic balls that brewers place inside a beer can. When the can is opened, the widget triggers a rapid decompression of nitrogen gas throughout the drink. The flood of nitrogen gas produces tiny bubbles, giving the beer its smooth and creamy texture.

Devereux, who was supervised by William Lee, a junior lecturer in the same department, said, "Our proposed alternative to the widget would consist of an array of cellulose fibers of approximately three square centimeters. Our research suggests that stout could be made to foam using an array of fibers in 30 seconds, which is the time it typically takes to pour a glass of stout."

Inside of a nitrogen-enriched stout, pockets of air trapped inside cellulose fibers become seed bubbles that trigger nucleation (the formation of additional bubbles). As nitrogen and carbon dioxide diffuse through the walls of the fibers, the seed bubbles grow. When each bubble reaches a certain length, it detaches and breaks off of the gas pocket and floats to the top of the liquid. This process is repeated until the gas dissolves completely into the stout.

Using a microscope, Lee and Devereux observed how stouts react to this process and how the bubbles formed inside the cellulose fibers. They discovered that the bubbling rate of stout beers was 20 times slower than the rate for most carbonated drinks.

Devereux believes this research could benefit champaign researchers too because "we have noticed that it is much easier to study nucleation in stouts than it is in champagne and other carbonated liquids, because of the slow rates of bubbles," says Devereux. He adds, "Stout beers may prove to be a useful model system in which to study nucleation in carbonated liquids."